Writing Formula Units of Ionic Compounds: Study with Video Lessons, Practice Problems & Examples

The formula unit of an ionic compound represents the simplest ratio of ions that results in a neutral charge, forming a crystal lattice rather than isolated ionic pairs. For example, sodium chloride (NaCl) consists of a 3D arrangement of sodium and chloride ions. When writing ionic compounds, identify the ions, determine their charges, and either cancel or crisscross the charges to derive the formula. For instance, aluminum nitride is AlN, while barium phosphate is Ba(PO₄)₂. Understanding these principles is essential for mastering ionic compounds and their properties.

Writing Formula Units of Ionic Compounds Concept 1

Now the formula unit represents the empirical, what we call the simplest ratio of ions in an ionic solid that combine to give a neutral charge. We're going to say here in reality an ionic solid doesn't exist as an ionic pair. So it's not just a positive ion and a negative ion combining. It's not that ionic pair, but instead it's a crystal lattice. Now, a crystal lattice is a 3D arrangement of several cations and anions together that form a stable pattern. So for example here, we have the ions of sodium ion and chloride ion. They have opposing charges, which makes them attracted to one another. That's why they're kind of combined together. In reality, though, the ionic compound is not just a single positive ion and a single negative ion. It's a collection of all of them together. This helps to make my crystal lattice. So in reality, this is my ionic solid. It's a bunch of positive ions and negative ions combined together. Now, if we were to take one of these positive ions and one of these negative ions, pull them out, that there would represent the formula unit, which is the simplest portion of my ionic solid. So here, it would be sodium chloride, which would be NaCl. So just remember, when we're talking about ionic pairs, that's just a formula unit of an ionic compound. It is not the true representation of that ionic solid. The true representation is a collection of those positive and negative ions together which form a crystal lattice.

Writing Formula Units of Ionic Compounds Concept 2

Here we have the rules for writing ionic compounds. Now step 1, we write the ions involved in the compound from the provided name. Step 2, you use these ions to write the formula of the ion compound. So if we take a look here, we have aluminum, nitride, and barium phosphate. Alright. So we're going to say aluminum is in group 3A. Because of that, its charge is plus 3 or 3 plus. So that's why you write it as Al³⁺. Nitride, "ide" would mean that it is a nonmetal, and it had its ending changed to "ide". Nitrogen is in group 5A, so its charge is 3 minus. Now if you don't quite remember the charges associated with the different groups of the periodic table, make sure you go back and take a look at my videos on the periodic table charges. Okay? That'll help you see all the elements of the periodic table and what are the common types of charges based on the location of the element. Now for this first one, when numbers in charges are the same, so here the numbers are 3 and 3. Okay? So the numbers are 3 and 3, they cancel out to combine the elements. So since they both have 3 in their charges, they just cancel out. So aluminum nitride will be AlN. Now barium phosphate. Barium is in group 2A, so its charge is 2 plus. Phosphate is one of our polyatomic ions. It's one of our common tetraoxides. Its formula is P₄O₁₃^3- Now, when the numbers in charges are different, here this is 2 and this is 3, then they crisscross. They crisscross to combine the elements. So what I mean by crisscross is that the 2 from here would come here, and the 3 from here would come here. That would give me, if we look, it'd give me Ba₃(PO₄)₂. So that would represent barium phosphate. These are the things you must do when given the name of an ionic compound. Write out its ions and then look to see, are the numbers in the charges the same? If so, they cancel out. Are they different? If so, crisscross them. Now doing this crisscrossing motion, we don't have to worry about crisscrossing the charges. Okay? So you're only crisscrossing the numbers. Doing this type of action takes care of the charges on them on their own, so just ignore them. Okay? So these would be our two formulas for these ionic compounds. Now that we've gotten down the basics, we'll move on to some problems and put what we just learned into practice.

Writing Formula Units of Ionic Compounds Example 1

Provide the formula unit for the compound formed by the following ions. So we have magnesium ion and sulfate ion. Alright. So remember, when the numbers in the charges are the same, they just simply cancel out. Mg²⁺ SO₄^2-, just combine the Mg with the SO₄. The charges completely cancel out, so ignore the 2, ignore the positive and negative charges. So it'd just be MgSO₄ as our newly created formula unit for ionic solid.

Provide the molecular formula for the following compound:Sodium dichromate

• A. NaCr₂O₇
• B. Na(Cr₂O₇)₂
• C. NaCrO₄
• D. Na₂Cr₂O₇
• E. Na₂CrO₄

Here’s what students ask on this topic:

What is a formula unit in an ionic compound?

A formula unit in an ionic compound represents the simplest ratio of ions that results in a neutral charge. It is the empirical formula of the compound, showing the smallest whole-number ratio of cations to anions. For example, in sodium chloride (NaCl), the formula unit is NaCl, indicating a 1:1 ratio of sodium ions (Na⁺) to chloride ions (Cl^-). This ratio forms a crystal lattice, a 3D arrangement of ions, rather than isolated ionic pairs.

How do you write the formula for an ionic compound?

To write the formula for an ionic compound, follow these steps: 1) Identify the ions involved and their charges. 2) Determine if the charges are the same or different. If the charges are the same, they cancel out. If different, crisscross the charges to balance them. For example, for aluminum nitride, Al³⁺ and N^3- have the same charge, so the formula is AlN. For barium phosphate, Ba²⁺ and PO₄^3- have different charges, so crisscross to get Ba₃(PO₄)₂.

What is the difference between a formula unit and a molecular formula?

A formula unit represents the simplest ratio of ions in an ionic compound, while a molecular formula shows the exact number of atoms of each element in a molecule. For example, NaCl is a formula unit for sodium chloride, indicating a 1:1 ratio of Na⁺ to Cl^-. In contrast, H₂O is a molecular formula for water, indicating two hydrogen atoms and one oxygen atom in each molecule.

Why do ionic compounds form crystal lattices?

Ionic compounds form crystal lattices because the electrostatic forces between oppositely charged ions create a stable, repeating 3D structure. This arrangement maximizes the attraction between cations and anions while minimizing repulsion, leading to a lower energy, more stable configuration. For example, in NaCl, sodium and chloride ions arrange themselves in a cubic lattice to achieve stability.

How do you determine the charge of an ion from the periodic table?

The charge of an ion can often be determined from its group number in the periodic table. For main group elements: Group 1A elements form +1 ions, Group 2A form +2 ions, Group 3A form +3 ions, Group 5A form -3 ions, Group 6A form -2 ions, and Group 7A form -1 ions. Transition metals can have multiple charges, so their specific charge is usually given in the compound's name.